Especially since it's not that expensive a feature anymore. There's actually a $15 Pi-like ARM board called the Orange Pi PC that does 4k, because apparently ARM media SoCs with 4k support are that cheap that you can put one in a bargain-basement board that's not even aimed at media applications. Admittedly it's restricted to 30 FPS at 4k, but still, I'm sure Apple could do better if they wanted to.